Glycolic acid can be produced by the reaction of aqueous formaldehyde and carbon monoxide in the presence of an acid catalyst. This reaction is often referred to as the “hydrocarboxylation” or “carbonylation” of formaldehyde. The formaldehyde reactant is generally prepared by well-known methods as a aqueous mixture that contains about 35 to about 70 weight percent formaldehyde.
The hydrocarboxylation reaction is catalyzed by strong acids, either heterogeneous or homogeneous in nature. When using a homogeneous catalyst, as exemplified by sulfuric acid, the production of a purified glycolic acid product requires the removal of the acid catalyst from the reactor effluent. The glycolic acid product, however, is an unusually strong carboxylic acid (pKa ˜3.8) and is capable of strong hydrogen-bonding and polar interactions with water. These properties make the separation and recycle of the hydrocarboxylation acid catalyst from the glycolic acid product difficult. For example, conditions that are useful for the extraction of the acid catalyst frequently will result in the coextraction of excessive amounts of glycolic acid or water with the acid catalyst. Moreover, for environmental and economic reasons, it is often desirable to isolate the strong acid catalyst in a form that permits its recycle to the hydrocarboxylation reactor.
One solution to recovering strong acid catalysts from aqueous glycolic acid reaction mixtures is to convert the acid catalyst to an easily separable or insoluble salt. For example, U.S. Pat. No. 2,153,064 discloses a process in which the crude glycolic acid effluent from a sulfuric acid-catalyzed hydrocarboxylation reaction is treated with calcium carbonate to precipitate the sulfuric acid as calcium sulfate. This inorganic salt must be separated and disposed of or sold as a low value by-product. In another example, described in U.S. Pat. No. 3,859,349, a sulfuric acid catalyst is removed the glycolic acid product mixture by absorption using a basic resin. The resin is regenerated by addition of aqueous ammonia, and the absorbed sulfuric acid is converted into ammonium sulfate, which is removed from the process. The ammonium sulfate may be sold as a fertilizer or otherwise disposed of, but is not recycled to the hydrocarboxylation reactor. Although these methods are effective for separating the acid catalysts from the glycolic acid product, they do not provide for the direct recovery and recycle of the acid catalyst to the formaldehyde hydrocarboxylation reaction. These limitations point to the need for a process in which a homogeneous acid catalyst for the hydrocarboxylation of aqueous formaldehyde with carbon monoxide can be recovered and optionally recycled in an efficient and cost-effective manner.